Industrial automation platforms including industrial robots, machine tools, coordinate measurement machines, semiconductor robots and test/measurement equipment, as well as military fire-control and communication systems such as radar, telescopes, deformable mirror/pointing systems and gun/laser pointing systems, typically employ closed-loop control of linear and rotary translators to achieve sub-micron/sub-microradian precision. In such systems, optical position transducers are used to measure linear and rotary displacements. To achieve the highest resolution, diffraction of coherent light (i.e. from a laser) with a precision grating is used to create the transducer signal. The wavefront diffraction angle sensitivity actually increases with the increasing resolution of the grating by sin−1(λ/d) where λ is the wavelength of light and d is the period of the grating.
Typically, the platforms have translators that are mounted in a serial kinematic arrangement that employ one linear translator for each component of translation. Alternatively, large turrets can be employed for the component of translation. In each case, a grating in the form of a long deformable tape provides precision reference information from which a read head can measure the sub-micron incremental displacements.
Referring to FIG. 1, a typical arrangement for a read head 12 and grating 16 is shown. A laser source 10 of a translator-mounted read head 12 generates a transmitted beam 14 of coherent light having a wavelength of about 800–1200 nm onto a reflective diffraction grating 16. The read head 12 moves in the x-axis direction of the translator and experiences lateral displacement relative to the reflective diffraction grating 16 when the translator is moved. The transmitted beam 14 is reflected by the reflective diffraction grating 16 to generate a first ordered pair (+1, −1) of a reflected beam 18. The reflected laser beam 18 is transmitted through a reticle 20 that has a grating pitch equal to the reflective diffraction grating 16. The −1 order of the of the transmitted beam 14 derived from the +1 order of the reflected beam 18 and the +1 order of the transmitted beam 14 derived from the −1 order of the reflected beam 18 interfere with one another.
The read head 12 has a photodiode array 22 located at the point of maximum interference between the +1 and −1 orders and decodes the light pattern to generate a quadrature output having two complex signals Z1(x) and Z2(x). The complex signals Z1(x) and Z2(x) have the form:Z(x)=A(x)ej(2πx−vdt)+B(x)  Eq. 1                where: Z, A and B are complex functions of the displacement x between the read head 12 and the grating 16;        the quadrature outputs are the real and imaginary components of        Z(x); and        x is the instantaneous spatial frequency of the grating structure and v is the relative velocity of the read head with respect to the grating.        
As seen in FIG. 2, the two quadrature signals Z1(x) and Z2(x) roughly vary sinusoidally with the orthogonal displacement (x) of the read head 12 with respect to the reflective grating 16. The photodiode array 22 combines the light intensity patterns such that Z1(x) and Z2(x) are displaced by a phase angle of 90°. By using A/D converters with N bit resolution to quantize the quadrature signals Z1(x) and Z2(x), displacements along the x-axis can be measured with a resolution of roughly 1 part in 2N+1. Without initializing the displacement at an absolute reference point, the displacement serves as a relative position with respect to an arbitrary starting point.
In addition to measuring incremental displacement, it is necessary for the control system to employ additional sensors that sense home (i.e., origin of coordinate reference frames) and end-of-travel limits. In more advanced systems, the position transducer (i.e., the read head 12) can be configured to provide absolute position information or to provide secondary tracks from which the home and end-of-travel limits can be detected.
As such, there is a need for a reflective diffraction grating that can be used with read heads to measure the displacement of industrial platforms. Typically, reflective diffraction gratings are manufactured using a “chrome-on-glass” process and selective etching using photolithographic techniques and “Zerodur” class having a coefficient of thermal expansion (CTE)<1 ppm/° C. These types of gratings are expensive and require relatively long manufacturing cycles. Furthermore, high tooling costs (e.g., masks) and high capital costs (e.g., vacuum and other sophisticated equipment employed in traditional semiconductor fabrication) are required. With these batch processes, the length of the grating is limited by the size of the chambers used. Therefore, these reflective gratings are expensive to fabricate.
Alternatively, it is possible to machine the gratings into a simple material substrate. The gratings are machined using microreplication or diamond machining techniques that require the substrate to meet broad and disparate material requirements. Accordingly, these machining processes are expensive and time consuming to perform and can suffer cyclic errors every microreplication rotation of the master.
Accordingly, there is a need for a system and method of producing a reflective grating which is quick and economical. Furthermore, there is a need for a process of fabricating diffraction gratings of variable length from centimeters to tens of meters long. The present invention addresses these needs by providing an inexpensive process of manufacturing reflective gratings that employs a laser to write the diffraction pattern onto a polished substrate in a roll-to-roll process thereby avoiding mask costs, machining and high capital investment. In alternate embodiments, the diffraction pattern can be directly written onto polished translator surfaces and/or bearing elements to reduce parts count and assembly time.